tipc: fix stale link problem during synchronization

Recent changes to the link synchronization means that we can now just
drop packets arriving on the synchronizing link before the synch point
is reached. This has lead to significant simplifications to the
implementation, but also turns out to have a flip side that we need
to consider.

Under unlucky circumstances, the two endpoints may end up
repeatedly dropping each other's packets, while immediately
asking for retransmission of the same packets, just to drop
them once more. This pattern will eventually be broken when
the synch point is reached on the other link, but before that,
the endpoints may have arrived at the retransmission limit
(stale counter) that indicates that the link should be broken.
We see this happen at rare occasions.

The fix for this is to not ask for retransmissions when a link is in
state LINK_SYNCHING. The fact that the link has reached this state
means that it has already received the first SYNCH packet, and that it
knows the synch point. Hence, it doesn't need any more packets until the
other link has reached the synch point, whereafter it can go ahead and
ask for the missing packets.

However, because of the reduced traffic on the synching link that
follows this change, it may now take longer to discover that the
synch point has been reached. We compensate for this by letting all
packets, on any of the links, trig a check for synchronization
termination. This is possible because the packets themselves don't
contain any information that is needed for discovering this condition.

Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/link.c

@@ -1330,6 +1330,7 @@ static int tipc_link_proto_rcv(struct tipc_link *l, struct sk_buff *skb,
 	u16 peers_snd_nxt =  msg_next_sent(hdr);
 	u16 peers_tol = msg_link_tolerance(hdr);
 	u16 peers_prio = msg_linkprio(hdr);
+	u16 rcv_nxt = l->rcv_nxt;
 	char *if_name;
 	int rc = 0;
 
@@ -1393,7 +1394,7 @@ static int tipc_link_proto_rcv(struct tipc_link *l, struct sk_buff *skb,
 			break;
 
 		/* Send NACK if peer has sent pkts we haven't received yet */
-		if (more(peers_snd_nxt, l->rcv_nxt))
+		if (more(peers_snd_nxt, rcv_nxt) && !tipc_link_is_synching(l))
 			rcvgap = peers_snd_nxt - l->rcv_nxt;
 		if (rcvgap || (msg_probe(hdr)))
 			tipc_link_build_proto_msg(l, STATE_MSG, 0, rcvgap,

net/tipc/node.c

@@ -1079,7 +1079,7 @@ static bool tipc_node_check_state(struct tipc_node *n, struct sk_buff *skb,
 	u16 exp_pkts = msg_msgcnt(hdr);
 	u16 rcv_nxt, syncpt, dlv_nxt;
 	int state = n->state;
-	struct tipc_link *l, *pl = NULL;
+	struct tipc_link *l, *tnl, *pl = NULL;
 	struct tipc_media_addr *maddr;
 	int i, pb_id;
 
@@ -1164,12 +1164,20 @@ static bool tipc_node_check_state(struct tipc_node *n, struct sk_buff *skb,
 
 	/* Open tunnel link when parallel link reaches synch point */
 	if ((n->state == NODE_SYNCHING) && tipc_link_is_synching(l)) {
+		if (tipc_link_is_synching(l)) {
+			tnl = l;
+		} else {
+			tnl = pl;
+			pl = l;
+		}
 		dlv_nxt = pl->rcv_nxt - mod(skb_queue_len(pl->inputq));
 		if (more(dlv_nxt, n->sync_point)) {
-			tipc_link_fsm_evt(l, LINK_SYNCH_END_EVT);
+			tipc_link_fsm_evt(tnl, LINK_SYNCH_END_EVT);
 			tipc_node_fsm_evt(n, NODE_SYNCH_END_EVT);
 			return true;
 		}
+		if (l == pl)
+			return true;
 		if ((usr == TUNNEL_PROTOCOL) && (mtyp == SYNCH_MSG))
 			return true;
 		if (usr == LINK_PROTOCOL)